Note 6 - Fair Value Measurements - Estimated Fair Value of Senior Notes (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Senior Notes, Carrying Amount	$ 987,272	$ 986,597
Reported Value Measurement [Member]
Senior Notes, Carrying Amount	987,272	986,597
Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	938,895	1,018,702
Senior Notes 5.625% Due February 2020 [Member]
Senior Notes, Carrying Amount	248,344	247,853
Senior Notes 5.625% Due February 2020 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	248,344	247,853
Senior Notes 5.625% Due February 2020 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	256,911	261,991
Senior Notes 5.5% Due January 2024 [Member]
Senior Notes, Carrying Amount	248,685	248,585
Senior Notes 5.5% Due January 2024 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	248,685	248,585
Senior Notes 5.5% Due January 2024 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	251,829	263,617
Senior Notes 6% Due January 2043 [Member]
Senior Notes, Carrying Amount	490,243	490,159
Senior Notes 6% Due January 2043 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	490,243	490,159
Senior Notes 6% Due January 2043 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	$ 430,155	$ 493,094